The invention relates generally to releasable mounting arrangements for gauging sensors. More particularly, the invention concerns a quick connect/disconnect arrangement for a portable gauging sensor wherein the sensor may be mounted with high mechanical repeatability at any number of positions on a gauging fixture relative to a workpiece to be gauged.
Traditionally, gauging systems for monitoring preselected dimensional characteristics of manufactured workpieces have used customized checking fixtures or ring gauges to measure such dimensional qualities as gap and flush dimensions. The ring surface of such a gauging fixture is machined to precisely reflect the exterior dimensions of a part and its flushness and gap with respect thereto. In operation of such a traditional part inspection system, the actual dimensions of a part have been checked relative to the gauging fixture surface with feeler gauges or vernier calipers. Such tools are conventionally manipulated by a human inspector and are highly subject to human error. In addition, they actually contact the part and fixture ring surface, thereby imparting part deformation or damage and ring surface wear.
Some recently introduced gauging systems now use portable, hand-held sensors to gauge a part held in a relatively simple fixture. Such systems were conceived to permit flexibility of measurements with rapidly changing workpiece designs. Additionally, use of portable gauging sensors, while enabling continued use of existing gauge fixtures already purchased, avoids the need for expensive, precisely toleranced ring surfaces. Rather, with a portable sensor in the newer type of gauging system, all that is required is a plurality of sensor mounting pads or "landing pads" precisely positioned at various points on the somewhat cruder fixture at key measurement positions relative to the part to be gauged.
A known example of such a gauging system utilizing a hand-held, portable gauging sensor is disclosed in U.S. Pat. No. 4,640,014 to Thomas. Thomas teaches a hand-held, portable sensor with two contact-type probes and associated transducers for measuring gap and flush at predetermined points about the periphery of a workpiece to be gauged. To locate the portable sensor at the predetermined points about the gauging fixture surface, Thomas teaches the use of a pin projecting from the sensor housing which mates with a bore provided in a channel of the locating block holding the sensor housing.
As is known in the pertinent art, to mechanically restrain a rigid object in three translational axes and three rotations about those axes, it is necessary and sufficient to fix three points along one translational axis, two points along a second orthogonal axis and one additional point along a third axis, all three axes being mutually orthogonal. Hence, it will be seen that the sensor mounting arrangement disclosed by the Thomas patent is spatially overconstrained. The chamfers running along two sides of both the sensor housing and its landing pad each constrain the mating of the two devices in first and second orthogonal axes. Assuming these four mating surfaces are both machined and ground flat, then two points of contact will be created on both sides of the chamfered channel. This aspect of the Thomas disclosed approach would require a high level of expensive metal working precision to allow these four surfaces to properly mate. Additionally, the pin/bore combination constrains the mating along a third orthogonal axis as well. The placement of the pin and bore relative to the mating channels would also require a high level of machining precision. This is due to the fact that there are three unnecessary points of contact along one axis and one unnecessary point of contact along another.
Mechanical repeatability, which is the measure of how well the sensor performs when it is mounted and remounted upon the same mounting or landing pad with the same part and position, is somewhat compromised in the Thomas disclosure. The repeatability of the pin/bore mating arrangement is limited by "slip fit" requirements. Machining/grinding practices dictate that at least 0.001-0.002 inches of clearance be kept between the inside diameter of the bore and the outside diameter of the mating pin. This results in a minimum positioning uncertainty of 25 to 50 microns. In addition, this level of "slip fit" is very tight, subject to binding, and requires the operator to carefully insure that the axes of the pin and bore are precisely aligned prior to mating. Additionally, repeated insertions and removals will impart significant wear to both the pin and bore over time, resulting in degraded repeatablilty.
Finally, in approaches such as taught in the Thomas patent, the operator must have a steady hand and be capable of exerting a constant pressure on the sensor during the measurement cycle. This is not only ergonomically unpleasing from the standpoint of operator fatigue, but also implies that the operator is critically linked to the quality of the measurements.
In many circumstances, it has been found desirable to use a non-contact optical sensor as a portable measurement device for movement from check point to check point about the periphery of the workpiece being gauged. Such a truly portable vision sensor would allow measurements such as gap and flushness to be acquired without contacting either the workpiece or the ring surface of the gauging fixture thus eliminating problems of deformation, damage, and wear caused by contact with sensor elements. However, whether the gauging system utilizes contact or non-contact probe elements, the need for an improved repeatable mounting arrangement for the portable sensor at any of the landing pads mounted to the gauging fixture is required.